This invention relates generally to protection circuits for electronic circuitry.
An electronic switching module includes a plurality of switching channels actuated by a controller or electronic processor. Each of the switching channels includes a power driver or switching transistor for coupling and decoupling electrical power to a load or accessories. The switching module is often located in a host module wherein other electronics, such as a microcomputer, are located. In automobile applications in particular, failures in either the switching module, host module, or accessory cannot be tolerated.
Protection circuits are known for protecting against overload or short conditions by detecting either excessive voltage drops across the switching transistor or measuring excessive current through a switching transistor. Both forms of detection occur when either the overload or the short forces the switching transistor out of saturation. A separate protection circuit is required for each switching transister. In response to the detection, the switching transistor is shut off. Examples of devices of this type are disclosed in U.S. Pat. Nos. 3,578,983; 3,591,832; 3,599,042; and 4,158,866.
The inventors herein have recognized numerous disadvantages of the prior approaches. In automobile applications, in particular, more than short circuit or overload protection is needed. Any degradation in the performance of either the switching module or the host module should be avoided. For example, considering the high density packaging and environment of these modules, unusual heat dissipation of the switching transistor may result in massive module failures. Thus, short circuit or overload protection as described above with respect to the prior approaches is not adequate. Further, application of low voltages to some automobile accessories, such as solenoids, may either fail to actuate the accessory or damage the accessory.
An additional disadvantage of the prior approaches is that a separate protection circuit is required for each switching transistor. The number of additional components and interconnections required to protect a plurality of switching transistors is therefore prohibitive in a module having a high density of electronic components.